This invention relates to a method for overwriting a phase change optical recording medium, and an optical recording medium which is overwritten by such method.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. The phase change optical recording media are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Be--Te system and Ge--Sb--Te system which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds have been investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228. Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992. In addition to the optical recording media of phase change type wherein chalcopyrite compounds are used, optical recording media of phase change type wherein AgSbTe.sub.2 phase is formed with the crystallization of the recording layer is disclosed in JP-A 267192/1992, 232779/1992, and 166268/1994.
In the optical recording media of phase change type, a groove is formed in the substrate for the purpose of tracking, and address information is sometimes loaded in the groove. It has been common to form record marks in the groove, or in the region (land) formed between the adjacent grooves. Recently, land/groove recording has been proposed wherein both the land and the groove are used for the recording tracks to thereby reduce the track pitch and enable a high density recording (Japanese patent Publication No (JP-B) 57859/1988).
When the recording track pitch is reduced as in the case of the land/groove recording, spot of the laser beam used for the recording/reproduction will extend into the adjacent track to result in a phenomenon wherein the signals in the adjacent track are erased in the overwriting (cross erase). Crosstalk in the reproduction is also increased.
The cross erase and the crosstalk may be reduced by using a smaller laser beam spot, and more specifically, by shortening the laser wavelength or increasing numerical aperture (NA) of the optical system. At present, however, a semiconductor laser with a short laser wavelength is insufficient in life, power, cost, and the like, and use of an optical system with a large numerical aperture (NA) is associated with the problem of reduced skew margin of the optical recording medium and reduced focal depth. For example, JP-A 204686/97 describes that the skew margin is proportional to the .lambda./[t.multidot.(NA).sup.3 ], and Handbook of Magneto-Optical Disc Production (1991, issued from Science Forum) describes that the focal depth is proportional to .lambda./[2(NA).sup.2 ] when .lambda. stands for wavelength of the recording/reproducing light and t stands for the medium thickness. When the skew margin is small, deformation of the beam spot of the recording light and the reproducing light will be increased upon tilting of the medium to result in significantly increased cross erase and the crosstalk. When the focal depth is shallow, the beam spot will be ambiguous when the focus servo is unstable or the medium has minute deformations, and the cross erase and the crosstalk will also be increased in such case. To reserve the skew margin in the case when an optical system with a large NA is used, the medium thickness may be reduced as described in JP-A 9-204686. However, the skew margin is inversely proportional to NA.sup.3 and to the thickness t of the substrate as described above, and when NA is considerably large, the thickness of the resin substrate of the medium should be greatly reduced, and in such a case, the substrate is likely to suffer from minute deformation to require a deep focal depth, and hence, a reduced NA.
As described above, the recording/reproducing wavelength and the NA that enable a stable operation should be adequately selected in correspondence with the state of the art in both economical point of view and in technical point of view such as reservation of the skew margin. In addition, a high density recording should be realized by reducing the recording track pitch, and influence of the cross erase and the crosstalk should be minimized as far as possible.
In view of the recent operational use of DVD-ROM (read only memory DVD), an optical recording medium should be able to handle an animation at a practically acceptable level. Accordingly, an optical recording medium should have an increased volume as well as improved transfer rate. More illustratively, an optical recording medium should have a transfer rate which is at least higher than the average transfer rate of the DVD-ROM, namely, at least 3.5 Mbps (linear velocity of 3.47 m/s). In the case of a recordable system, the transfer rate should be at least twice the transfer rate of a reproduction-only system, and the transfer rate should be as high as at least 20 Mbps to realize a picture quality required in business. The transfer rate can be improved by increasing the linear velocity and the recording density of the medium. Such high linear velocity, however, results in unstable tracking servo to invite increased error, and in extreme cases, the laser beam spot goes off the recording track to render the recording/reproducing operation impossible. When an optical system with a large NA is used to increase the recording density, the focal depth will be shallow as described above, and in such a case, the substrate is required to have a reduced thickness to reserve a sufficient skew margin, and the substrate may then suffer from increased deformation, and hence, unstable servo signals and tracking difficulties. Therefore, increase in the tracking servo signal is required in the medium of large volume and high transfer rate.